Polypropylene and polyethylene homopolymers and copolymers are produced in a variety of reactors, such as loop polymerization reactors (e.g., slurry loop reactors).
Within the reactor, it is often desirable to increase the temperature within the reactor to improve catalyst efficiency, increase product crystallinity and/or increase the energy efficiency to flash liquid components from the solid product. It may also be desirable to increase the hydrogen concentration of the reactor composition (e.g., slurry) to reduce the molecular weight of the resultant product. However, these adjustments also increase the vapor pressure of the composition, which may cause gas bubbles to form if the vapor pressure is sufficiently close to, or exceeds, the operating pressure of the reactor. The bubbles can cause operational difficulties, such as errors in density measurements, diminished slurry circulation, or loss of slurry circulation resulting in reactor shutdown.
However, the vapor pressure of the slurry is difficult to estimate because conditions within the reactor (e.g., concentrations of components) are difficult to determine. Specifically, it is difficult to achieve mass balance of the hydrogen and “unknown” components of the liquid vapor pressure due to unmetered losses of hydrogen and other components and low accuracy of hydrogen reaction rate estimates. Moreover, the nature of the reaction slurry (solid reactive polymer particles dispersed in the reaction liquid) make it extremely difficult to reliably sample and measure component concentrations directly.
When a bubbling condition limit is reached or even approached, measures can be taken to restore or retain the slurry in its non-bubbling condition. One approach is to reduce the slurry temperature. Such reduction can be effected by reducing the temperature of, or increasing the flowrate of, a cooling liquid that surrounds the reactor, e.g., in an external cooling jacket surrounding a leg of the reactor loop. U.S. Pat. No. 7,678,341, incorporated herein by reference for this purpose, discloses a process for cooling polymerization reactors in the preparation of polyolefins.
WO/2001/082008 discloses a method for monitoring signals associated with downstream process equipment, e.g., electrical current, voltage or frequency signal, hydraulic pressure signals, or pneumatic pressure signals, to detect transients present in the signal that infer an associated change in product quality. An upstream process parameter can be adjusted in response to the inferred change in quality of the product, e.g., polypropylene.
U.S. Pat. No. 6,301,546 discloses detecting and monitoring changes in the properties of a fluidized bed of catalyst solids used to make synthesis gas which measures the magnitude of differential pressure fluctuations taken at different levels of bed elevation. Fluctuations in the pressure difference measured across a vertical section of the bed are related to the size of vapor-solid “bubbles” in the fluidized solids and can be used to track changes in the bed.
U.S. Pat. No. 6,718,234 discloses a system for online inference and control of physical and chemical properties of polypropylene and its copolymers. The system uses models for the inference of physical and chemical properties that are not continuously measured and relevant models to control these properties. Controlled variables include the power of the circulation pump, the opening of the valve that controls the temperature of each loop reactor, and the difference between the reactor temperature and the bubble point of the liquid within each loop reactor.
However, to date, there are no reliable analytical methods to detect bubbling within the reactor so that measures can be taken to limit or eliminate it. As such, to err on the side of caution, polymerization reactors typically must operate at pressures well above the vapor pressure of the slurry liquid circulating through the reactor.
That said, the desire to reduce catalyst cost and increase crystallinity and melt flow rates (MFRs) of the homopolymers and copolymers (e.g., impact copolymers) creates an incentive to operate the reactors at higher reactor temperatures and increased hydrogen concentrations. Moreover, the desire to increase plant capacity to reduce the capital and operating cost per unit of production creates an incentive to raise the reactor operating temperature. However, these adjustments tend to increase vapor pressure of the circulating slurry in the reactor.
That said, it would be useful to provide a method for determining when a slurry liquid circulating through a reactor has reached or surpassed a point at which bubble formation occurs to provide improved product and reactor efficiency while avoiding excessive bubbling or near-bubbling conditions.